Aircraft engine assemblies produce high noise levels due to the high airflows through inlets, rotating stages and exhaust nozzles, etc. To reduce such noise levels and comply with noise regulations governing commercial aircraft, high bypass type aircraft engine assemblies incorporate acoustic panels in various locations of the engine, such as in the inlets of engine nacelles, thrust reversers, etc. These acoustic panels, sometimes referred to as acoustic treatments may comprise a septumized cellular, or “honeycomb” core oriented, or “sandwiched” between a perforated inner skin and a non-perforated outer skin or facesheet. The skins or facesheets may comprise a metal such as aluminum or a composite material, and the honeycomb core may comprise a metal, a ceramic, or a composite material.
Acoustic sandwich panels include a core sandwiched between two liner sheets. One of the liner sheets is perforated while the other sheet is not perforated. The core provides bulk (e.g. separates the inner and outer facesheets) and defines a plurality of cavities. Apertures defined by the perforated liner sheet fluidly couple the cavities ambient environment. Therefore, when air flows across the perforated liner sheet of an acoustic sandwich panel, the cavities in the core act as Helmholz resonators and attenuate the sound of the associated airflow. More specifically, core walls between adjacent cavities in the core act as acoustic septa to attenuate the sound of the airflow
Over the course of a useful life, acoustic panels may require localized repair or reconditioning, known as “rework” to restore their original structural strength and performance. Known reworking for acoustic panels involves splicing the honeycomb core, and using foaming adhesives to secure the reworked sections. However this type of rework can result in the blockage of honeycomb cells, or otherwise limit the cells comprising the honeycomb core, resulting in less than desired acoustic performance of the affected cells. While certain types of aircraft have allowable acoustic loss at acoustic panels within acoustic structures, regulations for other aircraft types may not permit structural rework of such areas, resulting in part replacement at significant cost. Further regulations may allow for the replacement of a portion of the acoustic panels if acoustic performance is restored. However, restoring acoustic performance for reworked acoustic panels and structures comprising acoustic panels, such as aircraft nacelles, thrust reversers, etc., has presented continuing challenges.